There is a critical need in the field of biomaterial science to develop simple methods for assembly of well controlled, biocompatible and functionalized biomaterial coatings. Numerous modification techniques have been developed with the aim of tailoring a material surface with desired bioactivity and biocompatibility, including chemical grafting of functional groups and physisorption of specific molecules. While these methods are effective for specific needs, they also present limitations with respect to complexity of processing, loss of bioactivity of biomolecules to be delivered and limited control of biodegradation.
The layer-by-layer (LbL) assembly technique offers an alternate strategy to form biofunctionalized surface coatings. Traditional LbL pioneered by Iler and Decher et al. (Iler, R. K. J. Colloid Interface Sci., 1966, 21, 569-594; Decher, et al Thin Solid Films, 1992, 210/211, 831-835; Decher, G. Science, 1997, 277, 1232-1237) is based on the alternate deposition of oppositely charged polyelectrolytes, forming interpenetrating layers of polymeric salts. The driving force for this LbL assembly is primarily electrostatic interaction, but the process can also involve charge transfer interactions, van der Waals interactions, hydrogen bonding, and short-range hydrophobic interactions (Kotov, N. A., NanoStructured Materials, 1999, 12, 789; Lojou, E.; et al, 2003, 20, 748-755). The procedure is facile, inexpensive, and very versatile. The coatings may be formed on virtually any substrate in almost any shape and size and generally do not require intensive chemical processing. Thus, it is possible to incorporate materials with desired functions into these coatings, including pharmacological drugs, growth factors, and signaling proteins (Jessel, N. et al Advanced Materials, 2003, 15, 692-695). These functionalities can either be one of the polyelectrolyte layers in the assembly or entrapped between layers with nanometer- or micron-scale control.
Fundamental and applied studies of LbL coatings in terms of biological applications include the fabrication of films engineered to promote or inhibit the attachment of cells (Elbert, D. L. et al. Langmuir, 1999, 15, 5355-5362; Serizawa, T. et al, Biomacromolecules, 2002, 3, 724-731), the immobilization of living cells (Chluba, J. et al. Biomacromolecules, 2001, 2, 800-805; Grant, G. G. S. et al. Biomed. Microdevices, 2001, 4, 301-306), the immobilization of active enzymes (Jin, W. et al. Chem. Soc. 2001, 123, 8121-8122; Lvov, Y. et al. Nano Lett. 2001, 1, 125-128; Tiourina, O. P. et al. Macromol. Biosci. 2001, 1, 209-214), and the sustained release of functional DNA (Zhang, J. T. et al. Langmuir, 2004, 20, 8015-8021).
In the last decade the use of silk fibroin as a biomaterial has expanded for studies in vitro and in vivo due to the unique combination of mechanical structural and biocompatible properties exhibited by this protein (Sakabe, H. et al. Sen-i Gakkaishi, 1989, 45, 487-490; Park, W. H. et al. Fibers Polym, 2001, 2, 58-63; Santin, M. et al. J Biomed Mater Res., 1999, 46, 382-389). Comprehensive studies of the mechanical properties and inflammatory response suggest silk fibroin as an important material option in the fields of controlled release, biomaterials and scaffolds (Meinel, L. Hofmann, et al. Biomaterials, 2005, 26, 147-155). Regenerated silk fibroin has been successfully processed into films, gels, electrospun fiber mats and 3-dimensional porous scaffolds (Min, B.-M. et al. Biomaterials, 2004, 25, 1289-1297; Kim, H. J. et al. Biomaterials, 2005, 26, 4442-4452). In addition, aqueous solutions of these proteins have been optimized recently (Kim, U. J. Biomaterials, 2005, 26, 2775-2785).
However, while silk fibroin materials are proven to have promising potential, a means for adequately controlling the assembly of silk fibroin coatings remain to be determined. The ability to control the formation of silk coatings having specified properties including defined thickness, surface chemistry, and structure is important for functionalizing protein-based biomaterial surfaces for applications such as medical device coatings and tissue engineering scaffolds. Further, a tightly controlled assembly process is a clear necessity for the development of pharmaceuticals, e.g. controlled release biomaterials. In addition, processes that can function in an all water mode offer important benefits to preserving the function of sensitive compounds, cells or other components that may be entrapped or entrained in the layers or devices.